Field of the Invention
The invention lies in the field of digital image processing in reproduction technology and pertains, more specifically, to a method of generating trapping contours at color edges in a print page. Such trapping contours serve for reducing the disturbances generated by misregistrations in the printing press, i.e. errors that arise when the colors are not printed on top of one another perfectly congruently but rather slightly shifted.
The artwork masters which are created for print pages in reproduction technology contain all the elements that are to be printed, including text, graphic objects, and images. In the case of electronically produced masters, these elements take the form of digital data. The data for an image are generated in that the image is scanned in a scanner by pixels and rows, each pixel is broken down into color components, and the color values of these components are digitized. The data for texts and graphic objects are generally generated directly in a computer by text processing and drawing programs. Depending on the output process that is to follow (i.e. output on a color printer or printing in a conventional printing press), the data for the page elements are generated in the color components red, green and blue (RGB) or in the colors of four-color printing, i.e. cyan, magenta, yellow and black (CMYK). The colors are printed on top of one another on the print medium (e.g. paper) in the color printer or the printing press.
Next, the digitized text, graphic objects, and images undergo diverse processing (e.g. color correction or modification in arrangement and shape, and final assembly into a print page) in a processing station in accordance with the specifications of a layout, while being visually monitored on a color monitor. The print page data are then converted into a data format which is suitable for output, for instance in the page description language PostScript, and stored. In this process, separate print page items of data, known as color separation data, are generated for each of the colors CMYK. The color separation data are recorded in high resolution on film material or directly on printing plates using a film or plate recorder. There also exist digital printing presses which work without plates. With these, the color separation data are transferred directly to the digital printing press and printed on the print medium there.
One problem that arises in printing technology is misregistration in the printing press, i.e. errors which occur when the colors are not overlaid perfectly congruently, but rather shifted relative to one another, in the printing units through which the paper continuously runs. These errors will now be demonstrated in a simple example wherein a red circle is printed on a black background. With reference to FIG. 1, subfigures 1a and 1b represent the color separations Y and M, the superimposition of which produces the red circle, and FIG. 1c represents the color separation K of the black background from which the area of the circle is missing. FIG. 2 represents the print result when the color separations Y, M and K are precisely superimposed in the printing press. The color separations Y and M completely fill the missing circle area in the color separation K. FIG. 3 represents the print result given misregistration. In this example, the color separation K has been shifted up and to the right relative to separations Y and M, so that the separations Y and M no longer completely fill the missing circle in separation K, and an unprinted stripe 1 emerges. Since the unprinted stripe 1 is the color of the paper, which is typically white, it contrasts sharply with the red and black colors and is perceived as obtrusive. The shifting of the color separations also produces a stripe 2 wherein all three separations Y, M and K are superimposed on one another. Since, in this case, the brighter colors Y and M are printed on the dark color K, this stripe 2 does not disturb the print result. For purposes of illustration, FIG. 3 represents a very large shift of the color separations. But the unprinted stripes that emerge given smaller shifts, such as occur in practice in printing operations, are also very obtrusive, and countermeasures must be taken to eliminate or reduce these errors.
One possibility for eliminating or reducing the above described disturbing effects caused by misregistration is to control the printing plate register or the paper feed in the printing press more precisely. But this is expensive and requires a large outlay, and it is not always possible to retrofit older printing presses with better control devices.
A second possibility consists in what is known as trapping in the preparing of the color separation data in the reproduction technique. Trapping is effected during the preparation of the color separation data. A trapping frame is generated in some color separations but not in others. The trapping of the correct color separations ensures that a sufficiently large overlap of the graphic objects in the color separations will still exist even given misregistrations in the printing press, and consequently obtrusive unprinted stripes cannot emerge. For which color separations a trapping frame must be created and for which not depends on the colors in the graphic objects relative to the colors in their immediate environment. As a general rule, the brighter colors are trapped vis-a-vis the darker colors. The outlines of the darker colors, which determine contour, are not changed, and the shapes of the graphic objects are thus subjectively maintained despite the overlapping of the color separations when they are printed on top of each other.
With reference to FIG. 4, there is shown a trapping frame 3 for the color separation Y from the example of FIG. 1; i.e., the circle is enlarged by the width w. When the circle in the separations Y and M is enlarged, but not the empty circular area in the separation K, then obtrusive stripes cannot emerge even when the color separations are shifted. The width w of the trapping frame 3 must be selected large enough to cover the maximum expected shift; i.e., the width is dependent upon the quality and age of the printing press that is used.
According to the prior art, the trapping frames are created by first analyzing which graphic objects are contained in a print page and where they adjoin one another. Depending on the type of objects and the colors on either side of a color boundary, it is then determined which of the objects will acquire a trapping frame and which color the trapping frame will take. The term graphic object refers to an area of arbitrary shape which is filled with color values in an arbitrary fashion. In the simplest case, the area is evenly filled with a constant color. But it can also contain a vignette, i.e. a continuous variation of the colors in a particular color gamut. But a graphic object can also contain scanned image data or a pattern of different color values. The type of color fill determines the type of graphic object, e.g. xe2x80x9cuniformly colored areaxe2x80x9d, xe2x80x9cvignettexe2x80x9d, or xe2x80x9cimagexe2x80x9d. The patent U.S. Pat. No. 5,113,249 teaches this kind of method for creating trapping frames according to the prior art. There, the type of graphic object is designated the xe2x80x9carea typexe2x80x9d, and depending on the xe2x80x9carea typesxe2x80x9d on either side of a color boundary, it is decided whether or not to create a trapping frame, which of the two area types to widen with it, and which color the trapping frame should take.
In the methods according to the prior art, the types of graphic objects on the print page and the boundaries of the objects must be determined. If the print page is defined in an object-oriented page description language such as PostScript, this can by accomplished by analyzing the page description. That type of method is described in the patent U.S. Pat. No. 5,295,236. If the print page is not given in an object-oriented description but rather as a matrix of pixels, then the graphic objects and their boundaries must be determined using a pattern recognition technique. These techniques involve large computing expenditures.
Additional computing power is consumed in the methods according to the prior art by the computation of the trapping frame itself. Once the decision is made as to which object is to be trapped, a frame of a specific width must be computed for the outline of the object, whereby the outline of a graphic object is generally a series of curves and straight lines. If the object has a complex shape as represented in FIG. 5, the computation of the trapping frame requires a large computing expenditure. The computing expenditure is further increased if only part of the boundary of the graphic object, and not the entire boundary, must be provided with a trapping frame. FIG. 6 represents an example wherein the graphic object 4 is surrounded by the color area 5 on part of its boundary and by the color area 6 on the rest of its boundary. Thus, a trapping frame 3 will be computed only for the part of the boundary that adjoins the color surface 5.
U.S. Pat. No. 5,386,483 (European patent EP 0 539 953) describes a method wherein a trapping frame is created with the aid of a window operator which is moved over a page row-by-row. A presumption upon which this method is based is that the page only contain areas which are uniformly filled with different colors (black copy image) and whose pixels are referenced by color numbers (region numbers). It is recorded in a color table which colors are assigned to which color numbers. The window operator determines whether the pixels within the window which are provided with different priorities have different color numbers. If so, provided that specific priority rules are satisfied, a new color number is assigned to the pixel in the center of the window. When the window operator has crossed the page a number of times, new frame-shaped color regions having new color numbers emerge at the boundaries of the original color regions. Lastly, the color table is expanded by the assigning of colors to the new color numbers. This method can only be applied when the page exclusively contains graphic objects which are areas of constant color.
All the prior art methods include the steps of distinguishing graphic objects of specific types on the page, computing the boundaries of these objects, and generating a respective trapping frame at the boundaries. The trapping frames thus acquire a constant color which is determined on the basis of the colors and/or the type of the adjoining objects. None of the methods in the prior art determines a trapping frame in a scanned image at expressed color contours, although disturbing effects can also emerge at such color contours as a consequence of misregistrations in the printing press.
It is accordingly an object of the invention to provide a method of defining a trapping frame on a print page, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which generates trapping frames and puts forward an effective and simple method by which a trapping contour is created in a page, which reduces or eliminates the disturbing effects created by registration errors.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of generating trapping contours in a page provided in the form of digital image data, for ameliorating disturbing effects of misregistrations in a printing machine. The method comprises the following method steps:
moving a window over the image data of the page pixel-by-pixel, wherein the window contains a center pixel and neighbor pixels premarked for further processing;
determining respective dark values for the center pixel and for the premarked neighbor pixels from color separation proportions;
determining a darkest color separation for the center pixel and for the premarked neighbor pixels;
comparing the dark values and the darkest color separations of the center pixel and of the premarked neighbor pixels for selecting particular pixels from the premarked neighbor pixels;
calculating a trapping color from the selected particular pixels;
comparing the color separation proportions of the center pixel with the corresponding color separation proportions of the trapping color and correspondingly modifying the color separation proportion of the center pixel.
The expression xe2x80x9cpixelxe2x80x9d as used herein should be understood as any picture element as used in digital picture data processing.
In other words, respective dark values are determined for the central picture element and the premarked neighboring picture elements from the color separation proportions; then
the darkest color separation is determined for the central picture element and the premarked neighboring picture elements;
particular picture elements are selected from the premarked neighboring picture elements by comparing the dark values and the darkest color separations of the central picture element and the premarked neighboring picture elements;
a trapping color is calculated from the selected picture elements;
a decision is made how to modify the color separation proportion of the central picture element by comparing the color separation proportions of the central picture element to the corresponding color separation proportions of the trapping color.
In accordance with an additional feature of the invention, the color separation proportions are weighted according to their subjectively perceived brightness for purposes of determining the dark values.
In accordance with another feature of the invention, the color separation proportions are weighted according to their subjectively perceived brightness for purposes of determining the darkest color separations.
In accordance with a further feature of the invention, the premarked neighbor pixels, only those pixels whose dark value is less than that of the center pixel and whose darkest color separation is not the same as that of the center pixel are selected.
In accordance with again an added feature of the invention, the color separation proportions of the trapping colors are calculated as the average values of the corresponding color separation proportions of the selected pixels.
In accordance with again another feature of the invention, a color separation proportion of the center pixel is modified only if the color separation proportion of the center pixel is less than a first threshold value, which is a function of the corresponding color separation proportion of the trapping colors, and if the first threshold value exceeds a second threshold value.
In accordance with yet another feature of the invention, if the black separation value of the center pixel exceeds a threshold value, the other color separation values of the center pixel are replaced by the corresponding color separation values of the trapping colors.
In accordance with a concomitant feature of the invention, the color separation proportions of the center pixel are modified by replacing them with corresponding color separation proportions of the trapping colors, which have been reduced by a reduction factor.
In summary, the inventive method does not presume knowledge of the graphic objects on the page or its type. It processes each color contour in the page the same way, regardless of whether this is a matter of a color boundary between graphic objects or a color contour within a scanned image.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for generating trapping contours in a print page, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.